1. Field of the Invention
This invention relates to improvments in a split type internal combustion engine including a plurality of cylinders split into two groups and operating in a split cylinder mode under engine low load conditions where one group of cylinders are supplied with fuel and fresh air and held operative and the other group of cylinders are supplied with neither fuel nor fresh air and held suspended.
2. Description of the Prior Art
It is generally known that internal combustion engines exhibit higher fuel combustion and thus higher fuel economy when running under higher load conditions. In view of this fact, split type internal combustion engines have already been proposed as automotive vehicle engines or the like subject to frequent engine load variations. Such split type internal combustion engines comprises a plurality of cylinders split into first and second groups, an intake passage provided therein with a throttle valve and being divided downstream of the throttle valve into a first branch leading to the first group of cylinders and a second branch leading to the second group of cylinders, an air stop valve provided at the entrance of the second branch for opening and closing the second branch, and control means responsive to engine low load conditions for closing the air stop valve to prevent fresh air from flowing into the second group of cylinders and for cutting off the supply of fuel into the second group of cylinders so as to place the engine in its split cylinder mode of operation. As a result, the other operative cylinders can operate with high loads, which results in high fuel economy.
Additionally, exhaust gases are re-introduced into the second branch to suppress pumping loss in the second group of cylinders during a split cylinder mode of operation. This attains further high fuel economy.
One difficulty with such conventional split type internal combustion engines is that during a split cylinder mode of operation, air discharged from the suspended cylinders is mixed with gases exhausted from the operating cylinders and discharged through its exhaust system. This causes a reduction of the temperature of exhaust gases passing through a three-way catalyzer provided in the exhaust system to spoil its exhaust emission purifying performance and also a reduction of the accuracy of the air-fuel ratio feedback control made by an oxygen sensor provided in the exhaust system.